I've been using the D&D Beyond Character Builder tool, and trying to create a Custom Attack to allow me to show the correct information for when I hit someone with my shield.  This would be an Improvised Weapon, and would be affected by the rules for dual wielding, no bonus damage since it's used in the off hand.

I can't figure out how to do it.  I've created the Attack Action, that went fine.  It shows up under "Other" with the box I can use to roll to hit, and when I use it, it shows up in the combat log correctly as a Shield Bash.  There doesn't seem to be any way to make the damage entry work, or add any information under "Notes".  As soon as I tell it that it uses a d4 the box shows up with the Str bonus added.  You can check the dual wield box, but there's no entry there to show which hand, the way a weapon does.  I get it that it's only an Improvised Weapon, but there isn't a checkbox for that.  It should be showing up under Bonus Actions.  All I can do is put all the information down under "Description" and it doesn't show unless I click on the entry for Shield Bash.  The whole point is to have the correct information on display so I don't have to look it up.

There isn't a single entry in the Homebrew listing that has what I want.  There are magic items with the name Shield Bash, but none of them do what I want.  I've had no luck at all trying to use the Homebrew Editor for anything.  It would be far too confusing for me if you tried to help me through the Homebrew Editor.  You'd have to take me through it step by step.

Can anyone tell me how to do what I want with the Character Builder tool?

 Yeah, the Dual Wield checkbox doesn't really do anything for Custom Actions, and the STR bonus can't be zeroed out. But you can set "Activation Type" to "Bonus Action" and it'll show up in the right area.

Edit: And yeah no Notes; try naming the action "Shield Bash (improvised)"

Edit2: Oh, the Snippet will at least let you set some readable info on the Actions page, like "An improvised attack with offhand shield that deals 1d4 damage."

Outstanding.  All of that works perfectly.  Now if I can get the damage box to work, all is well.  Thank you.

Alternatively, you could add a 1d4 bludgeoning weapon that you lack proficiency in, customize it to have Dual Wield checked and Weight Override 0, and rename it "Shield Bash". That attack should have the proper To-Hit and Damage.

Quote from Geann >>

Outstanding.  All of that works perfectly.  Now if I can get the damage box to work, all is well.  Thank you.

 You won’t be able to get it to not add Str to the damage.

The easiest thing to do would be to make a “Magic item” based on a Club, and name it “Shield.” (You can even give it the +2AC and replace the actual shield.) that you could Mark as “off hand” and then apply a -modifier that negates your PB. Or else, you could do it as a Custom feat that gets added directly to your character sheet.

Those are definitely more robust solutions to the problem, Sposta, but Geann said they wanted something that didn't involve the homebrew system, which is why I only offered customizations. Also I suggested using a non-proficient weapon since the attack is supposed to be Improvised, and I couldn't be sure if Club would fit the bill for their particular character (since so many classes get Simple Weapon proficiency.)

Custom actions cannot independently adjust attack and damage, and cannot be told to omit Ability Mod from the damage. Even if they just used a regular club they could adjust the To Hit with a negative modifier to offset PB, and the damage to do away with ability mod.

They can do it by customizing an existing weapon, but not with a custom action.